1. Field of the Invention
The present invention relates to an image forming apparatus utilizing electrophotography, such as a copying machine, a facsimile, a printer and the like. The invention more particularly relates to an image forming apparatus in which a toner image formed on a surface of an image carrier is transferred by a transfer device, provided in a position separated from the image carrier, to a recording medium being conveyed contacting and along a part of the circumferential surface of the image carrier facing the transfer device, and after the transfer the recording medium is separated from the image carrier by the bouncing force and the weight of the recording medium itself to be conveyed toward a fixing unit.
2. Discussion of the Background
Image forming apparatus utilizing electrophotography are known and found in such products as copying machines, facsimiles, printers, etc.
An example of a construction of such an image forming apparatus is schematically shown in FIG. 1. FIG. 1 illustrates a photoconductor 1 as an image carrier, which is drum-shaped in this example, a charger 2, optical lighting 3, carrying information of an original document from an optical writing unit utilizing a laser light or an exposure unit exposing an original document, a developing unit 4, a transfer sheet entrance guide member 5, a transfer sheet 6 as a recording medium, such as a transfer paper or a transparent film sheet, a transfer charger 7 capable of corona charging, which is arranged in a position separated from the photoconductor 1, a discharging/separating device 8, a cleaning unit 9, a discharging light 10, a conveying guide plate 11, and a fixing unit 12.
The photoconductor 1 may be alternatively formed in a belt shape, the belt-shaped photoconductor 1 being spanned around a plurality of support rollers so as to rotate, for example.
In FIG. 1, when an image forming operation starts, the photoconductor 1 rotates in a clockwise direction as indicated by an arrow in the drawing. After the charger 2 charges the surface of the photoconductor 1, the optical lighting 3 exposes the surface of the photoconductor 1 to form a latent image thereupon. The latent image is developed to a visible image by toner distributed from the developing unit 4. The image developed by the toner then reaches a transfer area facing the transfer charger 7, where the developed toner image is to be transferred to a transfer sheet 6.
The transfer sheet 6 is fed out from a feeding unit (not shown), and is guided by the transfer sheet entrance guide member 5 to the transfer area synchronized with formation and developing of the latent image on the photoconductor 1. The transfer sheet 6, is conveyed in contact along a circumferential surface of the photoconductor 1 in the transfer area, and charged by the transfer charger 7 with a charge having an opposite polarity to the toner, thereby transferring the toner image on the photoconductor 1 to the transfer sheet 6. The transfer sheet 6 is then discharged by the discharging/separating device 8 to separate the transfer sheet 6 from the surface of the photoconductor 1.
The transfer sheet 6 is then guided by the conveying guide plate 11 to the fixing unit 12, where the toner image is fixed to the transfer sheet 6 by a heat roller and a pressure roller of the fixing unit 12. Then, the transfer sheet 6 is exited to an exit part of the apparatus (not shown).
After the transfer of the toner image to the transfer sheet 6, the surface of the photoconductor 1 is cleaned by the cleaning unit 9 to remove residual toner therefrom and is discharged by the discharging light 10. Thus, an operation for forming an image is completed. When making multiple images, the above operation is repeated.
In such an image forming apparatus as described above, when transferring a toner image from the photoconductor 1 to the transfer sheet 6, the transfer sheet 6 needs to contact the surface of the photoconductor 1, and additionally maintain a sufficient contact area of the transfer sheet 6 with the photoconductor 1 along a circumferential surface of the photoconductor 1. When such a contact area is not sufficient, for example when the period of time during which the transfer sheet 6 is conveyed while contacting the photoconductor 1 becomes short, the conveying direction of the transfer sheet 6 becomes unstable after transfer of the image from the photoconductor 1, and, as a result, jamming of the transfer sheet 6 occurs. Further, because the period of time during which the image is transferred becomes short, it occurs that the toner image is not sufficiently transferred, resulting in deterioration of image quality.
The contact area of the photoconductor 1 and the transfer sheet 6 becomes larger if the diameter of the drum-shaped photoconductor 1 is made larger. When the photoconductor 1 is belt-shaped, the contact area is made larger by arranging the support rollers supporting the belt-shaped photoconductor 1 so as to increase the radius of the curvature of the belt at the position where the transfer sheet 6 contacts the belt and an image is transferred. As the contact area becomes larger, it becomes more difficult to separate the transfer sheet 6 from the photoconductor 1 after the image is transferred to the transfer sheet. Therefore, generally, when the contact area between the photoconductor 1 and the transfer sheet 6 is made relatively large, a separating device such as the discharging/separating device 8 is used to separate the transfer sheet 6 from the photoconductor 1, which increases complexity of construction and raises the cost of the apparatus.
On the other hand, if the diameter of the drum-shaped photoconductor 6 is made smaller, or the radius of the curvature of the belt-shaped photoconductor 1 at the point of the transfer is made smaller, separation of the transfer sheet 6 from the photoconductor 1 becomes easier because of the increased curvature of the photoconductor 1. In this arrangement, the transfer sheet 6 separates from the photoconductor 1 by a bouncing force and weight of the transfer sheet 6 itself, and the separating device becomes unnecessary.
However, with a small-diameter photoconductor 1 or a belt-shaped photoconductor 1 with a smaller radius of the curvature at the transfer point, the contact area of the transfer sheet 6 and the photoconductor 1 becomes insufficient. As described above, insufficient contact area causes unstable conveying of the transfer sheet 6 and insufficient image transfer, resulting in jamming of the transfer sheet 6 and deterioration of the image quality.
In more detail, image density decreases because of insufficient charging due to insufficient contact time. Further, image blurring occurs due to uneven charge across the transfer sheet 6 at the time of transfer. Uneven charge across the transfer sheet 6 is caused, for example, by uneven electric charge caused by dust on a wire electrode of the transfer charger 7 or uneven leaking of a charge through the transfer sheet 6 due to uneven moisture absorption of the transfer sheet 6. The uneven charge is generally eliminated if the contact area between the transfer sheet 6 and the photoconductor 1 is large enough and the contacting time is long enough at the time of transfer. The problem of image deterioration due to uneven charge caused by leaking of the charge through the transfer sheet 6 at the time of transfer is particularly significant under high humidity conditions because resistance of the transfer sheet 6 decreases due to increased moisture in the transfer sheet.